Any reason NOT to change default Comcast gateway password?

Comcast was out here investigating a frightfully slow internet response that developed and in the process I switched from their single to their dual channel gateway (modem+router). In going over the final details, the tech handed me a slip with the usual IP address and default credentials to manage the router. I remarked "Of course, I'll change the password immediately". His response was, no, don't do that, the first thing they tell us at training is to NEVER change the admin password on the router. Apparently, this prevents them from logging in to the router for customer service

Well, my jaw dropped, but I'm wondering if something has changed about the security on newer routers that would prevent someone on the 'net side of the modem from being able to access the router via the default login and password. Or was this just Comcast's way of making it easier on themselves?